Gas laser utilizing the negative glow in a cold cathode glow discharge tube



Filed Sept. 21, 1964 Feb. 24, 1970 J. SMITH E 3,497,327

GAS LASER UTILIZING THE NEGATIVE GLOW IN A CQLD CATHODE GLW DISCHARGETUBE v 2 Sheets-Sheet 1 FIG. 3 12 J. MW? BY as. sumo/1 ,Zz we f FiledSept. 21, 1964 GAS LASER UTILIZING THE NEGATIVE GLOW IN A COLD CATBODEGLOW DISCHARGE TUBE r 2 Sheets-Sheet 2 BY D- 6. J/MMd/VJ United StatesPatent 3,497,827 GAS LASER UTILIZING THE NEGATIVE GLOW IN A COLD CATHODEGLOW DISCHARGE TUBE James Smith, Redhill, Surrey, and Derrick GeorgeSimmons, South Merstham, Surrey, England, assignors, by mesneassignments, to US. Phiiips Corporation, New York, N.Y., a corporationof Delaware Filed Sept. 21, 1964, Ser. No. 397,731 Claims priority,application Great Britain, Sept. 23, 1963, 37,322/ 63 Int. Cl. H015 3/22US. Cl. 331-94.5

ABSTRACT OF THE DISCLOSURE A gas laser is disclosed which employs thenegative glow of a cold cathode glow discharge between a cathode and anexternal anode to produce a population inversion. Reflectors are alsoemployed at both ends of an envelope containing the cathode and anodeand a gaseous atmosphere therein to aid in producing a laser beam alonga path extending into the region occupied, in operation, by the negativeglow portion of the discharge.

This invention relates to lasers, or as they are also called, opticalmasers, which can be used to amplify or modulate light signals or togenerate light signals.

This invention relates particularly but not exclusively to gas laserswherein a glow-discharge may be maintained within a substantiallycylindrical tubular envelope, and wherein the laser beam extends inoperation along the envelope parallel to the longitudinal axis of theenvelope. In such devices the glow-discharge can be maintained with theaid of a thermionic cathode or with the aid of a cold cathode; thepresent invention is concerned with lasers having a cold-cathodeglow-discharge.

The invention provides a cold-cathode glow-discharge laser comprising anenvelope containing a gaseous atmosphere, said envelope also containinga cathode and an anode for cooperating to produce the glow discharge andhence population inversion in said atmosphere, the laser also comprisingmeans for producing a laser beam along a path extending into the regionoccupied, in operation, by the negative glow portion of the discharge.

The path of the laser beam may also extend into some other portions ofthe discharge space, e.g. into the dark spaces on either side of thenegative glow.

The negative glow may occupy a region adjacent a fiat surface of thecathode, alternatively the cathode may be tubular so that in operation ahollow-cathode effect is produced, the negative glow substantiallyfilling the interior of the tubular cathode through which the path ofthe laser beam extends.

The laser may comprise a plurality of aligned sections each arranged tocooperate with the laser beam and each comprising a separate anode and aseparate cathode. In such an arrangement barrier means may be providedbetween adjacent sections to inhibit the formation of a dis chargebetween the anode of one section and the cathode of an adjacent section.

The invention will be described with reference to the accompanyingdrawings in which:

FIGURE 1 illustrates a first embodiment,

FIGURES 2 and 3 illustrate a second embodiment,

FIGURES 4 and 5 illustrate a third embodiment,

FIGURE 6 illustrates a fourth embodiment of a laser according to theinvention.

Referring to FIGURE 1 a substantially cylindrical envelope 1 surroundsan electrode arrangement formed by a tubular cylindrical cathode 2 and anumber of anode rods 3. The anode rods extend through the envelope in 14Claims 7 a gas-tight manner so that their external portions formconnection pins and one or more similar rods 4 are electricallyconnected to the cathode 2 and also extend through the envelope 1.

The envelope 1 is preferably formed of glass having a coefiicient ofexpansion closely similar to that of the material of the rods 3 and 4 inorder that satisfactory gas-tight seals can be made between the rods andthe glass; for instance, if Kodial glass is used the rods suitably maybe made of an alloy including iron, nickel and cobalt. The cathode 1 hasa row of apertures 5 formed in it; an anode rod 3 is arranged adjacenteach aperture and an anode rod is also arranged at each end of thecathode, Electrically conducting discs (not shown) may also be providedon the ends of the anode rods inside the envelope in order to provideadditional areas of conductor to which the current discharge may takeplace in operation and to dissipate the heat generated. These discspreferably lie in a plane substantially normal to the rods.

At each end of the envelope is a Brewster-angle window 6 and inoperation the laser beam extends through these two windows through thetubular cathode in the region of the cathode glow. The envelope isfilledwith, for example, a gas mixture of neon and helium at a pressureof several millimetres, for example 10 to mm., of mercury.

When the anodes are connected to the positive terminal of a currentsource and the cathode is connected to the negative terminal of thesource then a discharge will be initiated and a negative glow will beformed of which the major part resides within the cathode tube. Thedischarge will be in the form of a plurality of paths; the end dischargepaths each extend from an end anode into the open end of the cathodetube and intermediate discharge-paths each extend from an anode throughthe adjacent aperture 5 to the interior of the cathode tube. Theprovision of several anodes tends to ensure that the glow isevenly-distributed along the interior of the cathode tube and thistendency can be greatly enhanced by connecting each anode through arespective current-limiting resistor to the positive supply terminal. Inan experimental device constructed in this manner the cathode tube had adiameter of 7 mm. and was 30 mm. long; the anode to cathode operatingvoltage was about volts. The anode rods each comprised 1.5 mm. diameterwire which was 30 mm. long with about 8 mm. inside the envelope fortaking current from the discharge. These rods were provided in additionwith anode discs of about 7 mm. diameter.

One important difference between lasers which utilize the negative glowand those known devices which utilize the positive column part of a glowdischarge is that, if it is required to increase the laser length by afactor of 2 and yet keep the gain per unit length constant, in the caseof the negative glow laser the applied voltage would remain constant butthe current would have to be doubled in order to keep the cathodecurrent density constant whereas, in the case of the positive columnlaser, the current would be held constant but the applied voltage wouldhave to be approximately doubled in order to keep the voltage gradientalong the discharge constant. Thus the negative glow laser is usually alow voltage, high current device and the positive column laser is a highvoltage, low current device. Typical values for a 1 metre device are 150volts, 1 amp for the negative glow laser and 1200 volts, 40 ma. for thepositive column laser.

The applied voltage necessary for initiation of the discharge in thenegative glow laser is dependent on the anode-cathode spacing and istypically a few hundred volts. It can be reduced if necessary by theincorporation of an auxiliary trigger electrode, situated close to thecathode and connected externally to the anode via a high value, forexample 1 megohm resistor. In the case of the positive column laser astarting voltage of several kilovolts is required unless use is made ofan auxiliary high frequency Tesla discharge.

In a device using a negative glow nearly all the anode tto cathodevoltage can appear between the cathode and the glowing ionised layer.Thus, in the device described \with reference to FIGURE 1, electronsentering the negative glow region will normally have much greaterenergies than those in the positive column of a positive column laser.

In gas lasers as hitherto designed, it has been usual to make theenvelope of a very pure material such as quartz, because ions from thepositive column glow impinge upon the inner surface of the envelope andrelease into the gaseous atmosphere impurities contained in the materialof the envelope: these impurities tend to poison the gaseous atmosphere.In a negative-glow tubular cathode laser, for example, as describedabove, the major part of the negative glow is confined within thecathode. If a substantially flat cathode is used instead, for example asdescribed below with reference to FIGURE 6, the major part of thenegative glow is confined closely adjacent the cathode. Thus, in boththese cases, ions from the glow are not so likely to impinge upon thewall of the envelope 1: this means that it is no longer necessary to usea material as pure as quartz for the envelope in order to preventpoisoning of the gas atmosphere within the envelope. More impurities maybe present in the envelope material without prejudicing the operation ofthe device.

As in most cold-cathode glow-discharge devices in use, material from thesurface of the cathode tends to be sputtered onto the adjacent surfaceof the envelope wall: because the tubular cathode has such a largesurface area and extends along the active portion of the laser, a largearea of the envelope inner Wall is covered with a layer of sputteredmaterial which in operation is continuously being added to and whichacts as a getter and tends to keep the gas atmosphere clean even whenthe tube is not operating. The area of this gettering surface is ofcourse very much greater than that which may be obtained in lasers usinga positive column glow. It is also diflicult to continually add to thegettering surface in a laser using a positive column glow.

FIGURE 2 illustrates a second embodiment in which a plurality of tubularcathodes 2 are arranged in line along the device. Each cathode 2 has aseparate connecting rod 4 and at each end of each cathode is an anode inthe form of an annular disc 7 supported by a connecting rod 3 as alsoillustrated in FIGURE 3. This arrangement provides for individualcontrol of each of a number of separate sections of the laser so thateach section may be used independently to switch or amplitudemodulatethe laser beam.

FIGURE 4 illustrates a third embodiment in which the anodes are again inrod form as in the embodiment illustrated in FIGURE 1 and may again beprovided with anode discs. These anodes co-operate with a plurality oftubular cathodes similar to those illustrated in FIG- URE 2. Thecathodes may be provided with apertures similar to those shown in FIGURE1, especially if the cathodes are fairly long. If they are short thenapertures are often not necessary. In order to prevent the anode of oneanode-cathode section of the laser from forming a discharge with thecathode of another section a barrier electrode 8 is interposed betweeneach two adjacent sections. As can be seen more clearly in FIGURE eachof these barrier electrodes is in the form of two flat annuli 9 joinedby two concentric cylindrical tube portions 10 and 11; a connecting rod1.2 secured to the cylindrical portion 11 extends through the wall ofthe envelope 1. If desired the cylindrical portion 11 can be omitted andthe rod 12 may then be secured to the outer surface of the portion 10.In operation each barrier electrode 8 is maintained at a potentialapproximately midway between the potentials of the cathodes and anodes;if as in the embodiment illustrated in FIGURE 1, the anode/cathodevoltage drops are in the region of 150 volts then the barrier electrodesmay suitably be maintained at 75 volts positive with respect to thecathodes by returning them to a suitable positive terminal on the supplysource. The barrier electrodes will then inhibit discharges betweenadjacent cathode-anode sections of the laser.

In an experimental arrangement using a tube as illustrated in FIGURE 4the tube comprised nine tubular cathodes 2 each with its associatedanode 3. The tube was filled with a mixture of parts helium and 1 partneon at a total pressure of 12 mm. of mercury. With an anode to cathodevoltage of 150 volts the total current was 2.25 amps; as each of thenine cathodes was 10 cm. long this gave a current of substantially 25millia-mps per centimetre of cathode length. At each end of the tube waspositioned a spherical concave mirror having a radius of curvature ofcm. these mirrors being spaced cm. apart. The laser beam produced bythis arrangement at a power of the order of several hundred microwattshad a wavelength of 1.153 microns.

A further experiment used the same arrangement but connections were onlymade to three of the cathodes and anodes so as to use, in effect, only athree-section tube; the total current was 350 milliamps, that is to sayapproximately 12 rnilliamps per centimetre of cathode length. The mirrorarrangement was the same as before as also was the wavelength of thegenerated light in the laser beam.

Experiments made using the same arrangement but with various gasmixtures having helium-neon ratios varying from 10:1 to 1000:1 at totalpressurees from 3 to 35 mm. of mercury also produced a laser beam of thesame wavelength.

For comparison it is useful to note that in positivecolu-mn tubes themixture is usually of the order of 10 parts of helium to 1 part of neonwith total pressure of the order of 1 mm. of mercury.

The embodiment illustrated in FIGURE 6 incorporates a flat cathode plate13. At low discharge currents the negative glow will be adjacent theupper surface of the plate; as the current is increased the glow mayextend to the other side of the plate. If desired, the lower side of theplate as viewed in the figure may be coated with a layer of insulatingmaterial in order to inhibit this spreading. The laser beam travelsalong the top of the cathode through the negative glow region andpreferably is of substantially rectangular cross-section. The cathodecan be in the form of a single plate as illustrated in FIGURE 6 or canbe composed of a number of separate sheets arranged in succession in asimilar manner to the separate cathodes of the embodiments illustratedin FIGURES 2. and 4 with, if desired, suitable barrier electrodesbetween them. Anode rods 3 are provided in a similar manner to theembodiments described previously and may again be provided with anodediscs.

As an example, in a specific embodiment constructed so as to resemblethat described with reference to FIG URE 6 the cathode comprised tenseparate sheets 13 of molybdenum each measuring 5 cm. long by 5 mm.wide. these sheets were arranged end to end 10 mm. apart, substantiallyin the same plane, and substantially along a common axis.

Two anode rods 3 were arranged to cooperate with each cathode. Theseanode rods each comprised 1.5 diameter wire which was 30 mm. long withabout 8 mm. inside the envelope for taking current from the discharge.These rods were provided at their ends inside the envelope with anodediscs of about 7 mm. diameter as described above with reference toFIGURE 1, these discs being approximately parallel to the cathode sheetsand about 6 mm. from them.

Barrier electrodes were provided approximately midway between each ofthe cathode plates. These electrodes each comprised two metal washers of15 mm. diameter provided at their centres with holes of 7 mm. diameter.These two washers were spaced apart with metal spacers 2.5 mm. thick soas to leave the, central holes clear. The electrodes were each providedwith supporting connecting rods which serve to connect them to terminalsoutside the Kodial glass envelope 1. The central holes were aligned soas to give, in operation, the laser beam a clear path through them whichpath included at least part of the regions occupied by the negative glowportions of the discharges from the cathodes.

The envelope -1 was provided with Brewster angle windows at its ends.Outside each of these windows was disposed a spherical concave slightlytransmissive multilayer dichroic mirror 14 in the usual manner. Thesemirrors were constructed for peak reflection at 1.153,u.

Best results were obtained when the envelope was filled with a mixturecontaining 0.5 to neon and 99.5 to 95% helium at a total pressure ofbetween and 80 mm. of mercury. The optimum total pressure was found tobe to 30 mm. Hg. At this total pressure and when using a mixture of 2%Ne and 98% He as the filling laser output at 1.153;]. was obtained whenthe total discharge current reached 100 ma. (two ma. per cm. of cathodelength) from a DC. power supply. When using a mixture of 99% He and 1%Ne at a total pressure of 27 mm. Hg, 1.7 mw. of laser output power wasobtained with a total discharge current of 1.1 ampere from a DC powersupply.

It was also found to' be possible to obtain about the same average laseroutput power using an A.C. power supply.

If the dichroic mirrors were replaced by spherical concave mirrorscoated with aluminum film it was also possible to obtain laser output at1.15 3a.

The barrier electrodes were found to be most useful in all theembodiments when the laser was initially operated, i.e. during the timewhen cleaning of the gas filling and cathodes was occurring by theaction of sputtering the surface layer of cathode material on the wallof the envelope. During this time they were connected to an electricpotential approximately midway between that of the cathodes and anodes.After this cleaning has progressed sufiiciently it was possible to leavethem electrically floating without adversely affecting the laseroperation.

The embodiments described are applicable to the production of continuouslaser beams and to production of pulsed beams. Pulsed beams may beobtained by applying sinusoidal A.C. or a pulse supply between theanodes and cathodes. If the discharge current supply is alternating insign the tube may be made to act elfectively as its own rectifier.

Although the embodiments described have incorporated spherical concavemirrors this is not necessarily so. These mirrors may normally bereplaced by any concave curved mirror which has the desired reflectance,or even by flat mirrors. A type of concave mirror which may be used, ifa beam which is substantially rectangular in cross-section is required,is the cylindrical concave mirror. It, for example, a mirror of thistype were incorporated in the embodiment shown in FIGURE 6 it shouldpreferably be placed so that its axis having infinite radius ofcurvature is substantially parallel to the shorter edge of the cathodeplate. Combinations of cylindrical concave and/or spherical concave and/or flat mirrors may also be used.

What we claim is:

1. A cold-cathode glow-discharge laser comprising an envelope containinga gaseous atmosphere at a pressure for sustaining a populationinversion, a cathode and an anode external to the cathode within saidenvelope producing a glow discharge and hence a population inversion insaid atmosphere, the negative glow portion of said discharge defining apath for a laser beam, said path extending into and along the regionoccupied, in operation, by the negative glow portion of the discharge.

2. A laser as claimed in claim 1 wherein the cathode is of cylindricalform, said path extending inside said cylinder.

3. A laser as claimed in claim 2 wherein the anode comprises an anoderod substantially normal to and not extending as far as the axis of thecylindrical cathode and situated outside the end of the cathode.

4. A laser as claimed in claim 2 wherein the anode comprises an anoderod substantially normal to the axis of said cylindrical cathode andsituated external to and along the body of the cathode, said cathodebeing provided with an aperture corresponding in position to that ofsaid rod.

5. A laser as claimed in claim 1 wherein the cathode comprises asubstantially flat plate, said path extending along and adjacent asubstantially flat face of said plate.

6. A laser as claimed in claim 5 wherein the anode includes an anode rodsituated substantially normal to and opposite said flat face.

7. A laser as claimed in claim 3 wherein said anode rod is provided withan anode disc at the end thereof inside the envelope and coaxial withthe path of said laser beam.

8. A laser as claimed in claim 1 comprising a plurality of cathodes andanodes situated along a path coextensive with the laser path.

9. A laser as claimed in claim 8 including barrier electrodes betweensaid cathodes.

10. A laser as claimed in claim 9 wherein said barrier electrodes aresubstantially annular and substantially coaxial with the laser path.

11. A laser as claimed in claim 1 wherein the gaseous atmospherecomprises a mixture of from 99.5 to 95% of He and from 0.5 to 5% Ne at atotal pressure in the range 10 to mm. of mercury.

12. A laser as claimed in claim 11 wherein the atmosphere comprisessubstantially 98 to 99% He and 2 to 1% Ne at a total pressure in therange 20 to 30 mm. of mercury.

13. A laser as claimed in claim 11 wherein the means for producing alaser beam comprises at least one concave multilayer dichroic mirrorhaving peak reflectance at a wavelength of substantially 1.153;.c andplaced facing another mirror with a light path between these two mirrorspassing through at least part of said region occupied, in operation, bythe negative glow portion.

14. A laser as claimed in claim 13 wherein the mirrors are eachsubstantially spherical concave multilayer dichroic mirrors having peakreflectance at a wavelength of substantially 1.153

References Cited Hartwick: Investigation of Gas Lasers, Semi-AnnualTechnicalReport AD289525, September 1962, 8 pages, page 5 relied upon.

RONALD L. W. BERT, Primary Examiner

